READV(2) Linux Programmer's Manual READV(2)NAME
readv, writev, preadv, pwritev - read or write data into multiple buf‐
fers
SYNOPSIS
#include <sys/uio.h>
ssize_t readv(int fd, const struct iovec *iov, int iovcnt);
ssize_t writev(int fd, const struct iovec *iov, int iovcnt);
ssize_t preadv(int fd, const struct iovec *iov, int iovcnt,
unsigned long pos_l, unsigned long pos_h);
ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt,
unsigned long pos_l, unsigned long pos_h);
DESCRIPTION
The readv() function reads iovcnt buffers from the file associated with
the file descriptor fd into the buffers described by iov ("scatter
input").
The writev() function writes iovcnt buffers of data described by iov to
the file associated with the file descriptor fd ("gather output").
The preadv() function reads iovcnt buffers from the file associated
with the file descriptor fd into the buffers described by iov ("scatter
input") at an offset defined with pos_l and pos_h.
The pwritev() function writes iovcnt buffers of data described by iov
to the file associated with the file descriptor fd ("scatter input") at
an offset defined with pos_l and pos_h.
The pointer iov points to an array of iovec structures, defined in
<sys/uio.h> as:
struct iovec {
void *iov_base; /* Starting address */
size_t iov_len; /* Number of bytes to transfer */
};
The file offset specified for preadv() and pwritev() is defined by the
parameters of pos_l and pos_h. The kernel concatinates the low 16 bit
of pos_h with all bits of pos_l to derive the file offset where the
read or write operations start.
The readv() and preadv() functions work just like read(2) except that
multiple buffers are filled.
The writev() and pwritev() functions work just like write(2) except
that multiple buffers are written out.
Buffers are processed in array order. This means that readv() and
preadv() completely fill iov[0] before proceeding to iov[1], and so on.
(If there is insufficient data, then not all buffers pointed to by iov
may be filled.) Similarly, writev() and pwritev() write out the entire
contents of iov[0] before proceeding to iov[1], and so on.
The data transfers performed by readv(), preadv(), writev(), and
pwritev() are atomic: the data written by writev() and pwritev() is
written as a single block that is not intermingled with output from
writes in other processes (but see pipe(7) for an exception); analo‐
gously, readv() and preadv() are guaranteed to read a contiguous block
of data from the file, regardless of read operations performed in other
threads or processes that have file descriptors referring to the same
open file description (see open(2)).
RETURN VALUE
On success, the readv() and preadv() functions return the number of
bytes read; the writev() and pwritev() functions return the number of
bytes written. On error, -1 is returned, and errno is set appropri‐
ately.
ERRORS
The errors are as given for read(2) and write(2). Additionally the
following error is defined:
EINVAL The sum of the iov_len values overflows an ssize_t value. Or,
the vector count iovcnt is less than zero or greater than the
permitted maximum.
CONFORMING TO
4.4BSD (the readv() and writev() functions first appeared in 4.2BSD),
POSIX.1-2001. Linux libc5 used size_t as the type of the iovcnt argu‐
ment, and int as return type for these functions.
NOTES
Linux Notes
POSIX.1-2001 allows an implementation to place a limit on the number of
items that can be passed in iov. An implementation can advertise its
limit by defining IOV_MAX in <limits.h> or at run time via the return
value from sysconf(_SC_IOV_MAX). On Linux, the limit advertised by
these mechanisms is 1024, which is the true kernel limit. However, the
glibc wrapper functions do some extra work if they detect that the
underlying kernel system call failed because this limit was exceeded.
In the case of readv() the wrapper function allocates a temporary buf‐
fer large enough for all of the items specified by iov, passes that
buffer in a call to read(2), copies data from the buffer to the loca‐
tions specified by the iov_base fields of the elements of iov, and then
frees the buffer. The wrapper function for writev() performs the anal‐
ogous task using a temporary buffer and a call to write(2).
BUGS
It is not advisable to mix calls to functions like readv() or writev(),
which operate on file descriptors, with the functions from the stdio
library; the results will be undefined and probably not what you want.
EXAMPLE
The following code sample demonstrates the use of writev():
char *str0 = "hello ";
char *str1 = "world\n";
struct iovec iov[2];
ssize_t nwritten;
iov[0].iov_base = str0;
iov[0].iov_len = strlen(str0);
iov[1].iov_base = str1;
iov[1].iov_len = strlen(str1);
nwritten = writev(STDOUT_FILENO, iov, 2);
SEE ALSOread(2), write(2)COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2002-10-17 READV(2)
